TCP segments are sent as internet datagrams. The Internet Protocol header carries several information fields, including the source and destination host addresses. A TCP header follows the internet header, supplying information specific to the TCP protocol. This division allows for the existence of host level protocols other than TCP.
Take a look at the TCP header format below.
Source Port: 16 bits The source port number.
Destination Port: 16 bits The destination port number.
Sequence Number: 32 bits The sequence number of the first data octet in this segment (except
when SYN is present). If SYN is present the sequence number is the
initial sequence number (ISN) and the first data octet is ISN+1.
Acknowledgment Number: 32 bits If the ACK control bit is set this field contains the value of the
next sequence number the sender of the segment is expecting to
receive. Once a connection is established this is always sent.
Data Offset: 4 bits The number of 32 bit words in the TCP Header. This indicates where
the data begins. The TCP header (even one including options) is an
integral number of 32 bits long.
Reserved: 6 bits Reserved for future use. Must be zero.
Control Bits: 6 bits (from left to right): URG: Urgent Pointer field significant
ACK: Acknowledgment field significant
PSH: Push Function
RST: Reset the connection
SYN: Synchronize sequence numbers
FIN: No more data from sender
Window: 16 bits The number of data octets beginning with the one indicated in the
acknowledgment field which the sender of this segment is willing to
accept.
Checksum: 16 bits The checksum field is the 16 bit one's complement of the one's
complement sum of all 16 bit words in the header and text. If a
segment contains an odd number of header and text octets to be
checksummed, the last octet is padded on the right with zeros to
form a 16 bit word for checksum purposes. The pad is not
transmitted as part of the segment. While computing the checksum,
the checksum field itself is replaced with zeros.
Urgent Pointer: 16 bits This field communicates the current value of the urgent pointer as a
positive offset from the sequence number in this segment. The
urgent pointer points to the sequence number of the octet following
the urgent data. This field is only be interpreted in segments with
the URG control bit set.
You can also check my other post on IP header fields:
https://admin-ahead.com/forum/general-discussion/ipv4-packet-header-fields-an-overview/Thank you for reading.
